solution: remove sudo:false usage from travis.yml

Travis has deprecated the usage of sudo:false in travis.yml.
See this blog post for more details: https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration
Also: https://docs.travis-ci.com/user/reference/trusty/#container-based-infrastructure

Re-license grant + suggested readme update

Signed-off-by: Christopher Hall <hsw@ms2.hinet.net>

Signed-off-by: Christopher Hall <hsw@ms2.hinet.net>

[DragonFlyBSD] fix for IPv4 mapping not supported

solution: Handle as for OpenBSD by not attemption to set this option on the socket
Signed-off-by: Christopher Hall <hsw@ms2.hinet.net>

solution: on DragonFly also define ZMQ_HAVE_FREEBSD
          in addition to ZMQ_HAVe_DRAGONFLY
Signed-off-by: Christopher Hall <hsw@ms2.hinet.net>

Add scripts to ease performance graph generation

 Fix build on SL6.9. Fix #3603. 

Problem: formatting errors introduced by #3601

Solution: fix them with make clang-format-diff

mingw thread name fix

Solution: Manually push/pop an exception registration record.
Only set the thread name with a debugger present

Problem:  include/zmq.h is not so much broken as inconsistent in

          how it declares the file descriptor (fd) of a socket
          in struct zmq_pollitem_t vs. struct zmq_poller_event_t

Solution:  Make include/zmq.h consistent in this matter

Problem: PR template is mostly about contributing guidelines

Solution: rename it to CONTRIBUTING.md so that it is shown when
creating a pull request or issue

Problem: redundant check in condition

Solution: remove it

Problem: Usage of 'if_nametoindex' not supported in Windows XP.

Solution: Add ZMQ_HAVE_WINDOWS_TARGET_XP check to src/udp_address.cpp.

 Problem: stream_engine use after free

Solution: Don't allow concurrent heartbeat timers

Problem: behavior of closing a socket registered...

...in a poller is not documented.
Solution: Document it.

Problem: incorrect ZMQ_MAX_SOCKETS default value in doc

Solution: update the doc with the correct default value

CMake: Install debug libraries for debug msvc builds

problem: ws_engine duplicate code from stream_engine

Solution: New class called stream_engine_base which is inherited by ws_engine, zmtp_engine and raw_engine.

Problem: build on centos6/ubuntu12 still broken

Solution: move the definition of __STDC_LIMIT_MACROS somewhere the test actually uses

Problems: build broken on centos6/ubuntu12, missing licenses and wrong copyright dates

Solution: add it

Solution: fix them